1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, or a light-emitting device each including an oxide semiconductor.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
Since light-emitting devices using light-emitting elements have high visibility, are suitable for reduction in thickness, and do not have limitations on viewing angle, they have attracted attention as display devices which are alternatives to CRTs (cathode ray tube) or liquid crystal display devices. Specifically proposed structures of active matrix display devices using light-emitting elements are different depending on manufacturers. In general, a pixel includes at least a light-emitting element, a transistor that controls input of image signals to the pixel (a switching transistor), and a transistor that controls the amount of current supplied to the light-emitting element (a driving transistor).
In recent years, oxide semiconductors have attracted attention as a novel semiconductor having high mobility as in the case of polysilicon or microcrystalline silicon and having uniform element characteristics as in the case of amorphous silicon. Oxide semiconductors can be deposited on a substrate which has a low strain point, such as a glass substrate, and can be used for a large substrate of the fifth generation (one side of which exceeds 1000 mm) or later generation. A light-emitting device in which the oxide semiconductor is used for a transistor in a pixel instead of a conventional semiconductor such as silicon or germanium is being put to practical use.
Patent Document 1 describes a TFT in which an oxide semiconductor is used as an active layer is used for driving an organic EL element. Patent Document 2 describes an organic electroluminescent display device in which an active layer of a thin film transistor is formed using an oxide semiconductor.